1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus such as a composite copying machine which can read an original image, generate a read signal, and form an image on a paper sheet in accordance with the read signal, and also can receive image data or font data from an external unit and form an image on a paper sheet in accordance with the data.
2. Description of the Related Art
Some electrophotographic image forming apparatus is of a digital scheme, in which an image converted into an electrical signal is formed as a latent image on a photosensitive drum with a laser beam. Such an image forming apparatus is being put into practice as a composite apparatus capable of performing operations based on, e.g., the copy mode of printing an original image read by a scanner and the printer mode serving as that of an output terminal of a computer.
In this image forming apparatus, a latent image formed on the photosensitive drum is developed into a toner image by the developing unit, and the toner image is transferred onto a paper sheet by the transfer unit. The image is then fixed to the paper sheet by the fixing unit. Through this process, an image is formed. The fixing unit generally employs a heat roller scheme constituted by a heat roller incorporating a heating element, and a press roller for applying pressure. In this scheme, a toner image is fixed to a paper sheet using heat and pressure.
In a digital copying machine for forming a latent image with a laser beam, with changes in heat and pressure in a fixing process, a toner image on a paper sheet is blurred and has an undesirable solid portion, posing problems in terms of image quality. In the copying machine, in particular, in the copy mode, since a laser beam is pulse-width-modulated, an image having many halftone portions, i.e., intermediate gradation portions, is produced as in an analog copying machine. In the printer mode, however, a binary monochrome image is produced. That is, toner images on paper sheets have different characteristics in the two modes. More specifically, in the printer mode, an image is generally expressed by one or more pixels. Laser beams for printing one or more pixels, e.g., two pixels and one pixel, have large pulse widths and relatively high optical energies in correspondence with two pixels and one pixel, as indicated by a curve (a) and a curve (b) in FIG. 1. In contrast to this, in the copy mode, an image is not necessarily expressed in units of pixels, but is generally expressed by not more than one pixel. Laser beams for printing not more than one pixel, e.g., a 2/3 pixel and a 1/3 pixel, have small pulse widths and relatively low optical energies in correspondence with the 2/3 pixel and the 1/3 pixel, as indicated by a curve (c) and a curve (d). For this reason, as shown in FIG. 2A, in the printer mode, a binary monochrome image is formed. In the copy mode, as shown in FIG. 2B, not only a binary image portion but also a halftone portion is formed. When the fixing characteristics in the two modes are compared, it is found that the fixing characteristics in the copy mode are inferior to those in the printer mode for the following reason. In the copy mode, as shown in FIG. 2B, toner particles are sparsely distributed on the paper sheet, and hence the bond strength between the toner particles and the paper sheet after a fixing operation using heat and pressure is low. In contrast to this, in the printer mode, toner particles are densely distributed on the paper sheet, and hence the toner particles soak into the paper sheet to be firmly bond thereto after melting, as shown in FIG. 2A. In this copying machine, if a fixing temperature is set in consideration of a halftone portion in the copy mode as the worst condition in terms of fixing characteristics, over-fixing occurs in the printer mode, resulting in blurring and formation of an undesirable solid portion of a toner image.
As described above, in an image forming apparatus having two operation modes, i.e., the printer mode of forming a latent image with a laser beam or the like and the copy mode of producing an image having a halftone portion obtained by pulse width modulation of a laser beam, if a fixing temperature is set in consideration of a halftone in the copy mode, over-fixing occurs in the printer mode. As a result, a toner image is blurred and has an undesirable solid portion.